1. Field of the Invention
The present invention generally relates to a multi-beam luminous source apparatus, an optical scanning apparatus, and an image formation apparatus; and especially relates to a multi-beam scanner for an optical scanning apparatus used as a writing system of a digital copier, a laser beam printer, and the like, for scanning a surface of a photo conductor and the like simultaneously with two or more optical beams, and for forming an image at high speed.
2. Description of the Related Art
Conventionally, a tandem system multi-color image formation apparatus has been available, wherein photo conductor drums for different colors are arranged along a conveyance route of an imprint object, toner images in corresponding colors are formed at image formation stations of the corresponding colors, and the toner images are superposed to form a color image in one pass. This has contributed to increasing the image formation speed. Recently and continuing, with improvements in the speed, a simple printing with an on-demand printing system is available, wherein high quality image formation with homogeneity of images from print to print is increasingly required.
Further, as a technique of raising the recording speed, there is a method of raising the rotational speed of a polygon mirror for deflecting. However, this method requires higher power consumption, which generates heat that deforms the housing for accommodating an optical scanning apparatus. The deformation causes a displacement of a scanning position on a photo conductor. Examples of the deformation include a registration (toner) error and inclination of a scanning line. For this reason, the rotational speed of the polygon mirror cannot be raised too much if the homogeneity of the quality of image from print to print is to be maintained.
On the other hand, there is a multi-beam scanner for raising the recording speed, hence raising the speed of the optical scanning apparatus. The multi-beam scanner scans a surface of a scanning object with two or more beams such that two or more adjoining lines are simultaneously recorded. In this way, the speed is raised without raising the rotational speed of the polygon mirror.
Here, if the rotational speed of the polygon scanner is R (rpm), the moving speed of an imprint object is V (mm/s), the number of planes is N, recording density is D (dpi), and the number of luminous sources is m, the rotational frequency R of the polygon scanner is expressed by the following formula.R=(60/N)×(D/25.4)×V/m
Here, the number of the luminous sources is desired to be such that the rotational speed of the polygon scanner becomes less than a speed at which the housing deformation due to heat becomes a predetermined tolerance. In this aspect, it is effective to employ a semiconductor laser array wherein two or more luminous sources are monolithically arranged at intervals of dozens of μm.
For example, Patent Reference 1 proposes a multi-beam luminous source apparatus wherein a sub scanning interval of beam-spot arrays constituted by two or more luminous sources is adjusted by rotationally adjusting a luminous source unit that includes a semiconductor laser array around an optical axis of an image-formation optical system. Further, Patent Reference 2 discloses an image formation apparatus wherein two or more lines are simultaneously formed by carrying out a package scan using a two-dimensional-array unit.
[Patent Reference 1] Japanese Patent No. 3670858
[Patent reference 2] JPA 2003-211728